Although starch-derived syrups and sugars, notably corn starch-derived dextrose and maltose syrups, are used on a large scale in the food industry, they fail to possess the sweetness of sucrose. The lack of sweetness limits their utility in the food industry. Fructose, the keto isomer of dextrose, is sweeter than dextrose, maltose and sucrose. Mixtures of dextrose and fructose are frequently comparable in sweetness to sucrose.
In the presence of a suitable catalyst, dextrose will convert to an isomeric ketose or keto sugar such as fructose. This catalytic reaction proceeds towards an equilibrium of the two sugars. Depending upon the isomerization conditions, the equilibrium is usually between about 45-55 parts fructose to about 55-45 parts dextrose. Solutions rich in either dextrose or fructose tend to assume the same equilibrium.
Heretofore, alkaline substances such as sodium hydroxide (e.g., see U.S. Pat. No. 2,354,664) and basic ion exchange resins (e.g., see U.S. Pat. No. 2,746,889) have been used in isomerization processes. Unfortunately, the alkaline isomerization processes develop undesirable by-products such as psicose, saccharic acid, inorganic residues, and color bodies (cf. U.S. Pat. No. 3,285,776). The formation of organic by-products reduces fructose yields and requires additional processing for their removal (e.g., see U.S. Pat. No. 3,383,245).
U.S. Pat. No. 2,950,228 by Marshall et al. discloses glucose isomerases useful in converting dextrose to fructose. Within recent years, a host of other glucose isomerases from divergent microbial sources have been proposed. Although the isomerases are more suitably adapted to isomerization processes, the use of isomerase preparations is not entirely free from disadvantages. It has been experienced that many of the isomerase preparations (e.g., Marshall, supra) are highly susceptible to inactivation at elevated temperatures. As a result, relatively low isomerization temperatures (e.g., well below 55.degree.C.) over a more prolonged period of time are generally required to complete the isomerization process. When relatively low isomerization temperatures are employed, microbial infestations and by-products become a problem. Attempts to increase the isomerase activity rate by employing elevated temperatures are generally frustrated by the requirement of excessive and uneconomical amounts of isomerase. Many isomerase preparations suggested heretofore require the presence of objectionable or poisonous cofactors (e.g., arsenates) to effectively conduct the isomerization process. An isomerase preparation capable of isomerizating dextrose to fructose at relatively high temperatures without requiring the presence of objectionable cofactors would be beneficial in correcting the aforementioned disadvantages.